JP2007106782A - Method for producing biaxially oriented polyester film - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a biaxially oriented film having excellent appearance because of slight coating defects by lowering a film temperature when applying a coating liquid to a film. <P>SOLUTION: The method for producing the biaxially oriented polyester film comprises applying a coating liquid to a film having ≤65°C film temperature and ≤1,000 μm film thickness by an inline coating method and then stretching the film by a simultaneously biaxially stretching method. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention is effectively used for optical films, surface protective materials, magnetic recording media, thermal transfer materials, electrical insulating materials, release materials, packaging materials, etc., especially optical films, surface protective materials, and other applications that require transparency. The present invention relates to a method for producing a biaxially stretched film.

Transparent plastic films such as polyester (PET, PEN, etc.), polycarbonate (PC), polymethyl methacrylate (PMMA), triacetyl cellulose (TAC), amorphous polyolefin (amorphous PO) are lighter in weight than glass. Because of its favorable properties such as being hard to break and being bent, it is used as a base material for liquid crystal displays (LCDs), plasma displays (PDPs), flat panel displays (FPDs), electronic paper materials, nameplates, and windowed films. Yes. Among transparent plastic films, biaxially stretched polyester film has excellent properties such as mechanical properties, electrical properties, dimensional stability, heat resistance, transparency, and chemical resistance, and is highly versatile and cost-effective. Since the merit has a great advantage, it is suitably used for such applications. However, the biaxially stretched polyester film itself has insufficient adhesion to other materials such as hard coat layer of AR film for flat panel display and mat layer of diffusion plate for backlight of liquid crystal display when used in each application. Therefore, methods for forming an easy-adhesion layer made of a polyester resin, a polyurethane resin, an acrylic resin, or the like on the surface of the polyester film have been proposed (Patent Documents 1 and 2). Moreover, since the polyester film used as the base material of the pachinko card has a drawback that it is easily charged, a coating liquid containing an antistatic agent is applied to form an antistatic coating film to prevent charging. The method has been put into practical use. As described above, by applying the coating liquid to the biaxially stretched polyester film, it becomes possible to add various required characteristics. However, in the film for the above-mentioned use, adhesiveness and antistatic property are simply imparted. However, it is not necessary that the coating film is uniformly formed by the coating liquid, and these performances need to be expressed uniformly.
JP 2000-246856 A JP-A-62-263237

  However, the above-described conventional technique has the following problems. The coating liquid applied to the biaxially stretched film may generate aggregates at a high temperature, and a large amount of aggregates may be generated due to the heat of the film when applied. For example, when the film in the process is thick and the film after heating is difficult to cool, the above aggregates are likely to be generated. In the case of bar coating due to this agglomerate, the solidified coating solution adheres to the surface of the bar, and in the case of die coating, the solidified coating solution adheres to the tip of the die. The coating is not uniformly applied, and appears as a coating defect. In the subsequent processing steps, a hard coat layer, a mat layer, or the like is not uniformly formed, and there is a problem in appearance in optical applications.

The present invention employs the following means in order to solve such problems. That is, the production method of the biaxially stretched polyester film of the present invention is
A method for producing a biaxially stretched polyester film, in which a coating liquid is applied to a film having a film temperature of 65 ° C. or less and a film thickness of 1000 μm or more by an inline coating method, and then the film is stretched by a simultaneous biaxial stretching method.
It is.

  The biaxially stretched polyester film produced according to the present invention exhibits an effect in excellent appearance and antistatic properties with very few coating defects while having the conventional advantages.

  Hereinafter, the method for producing a biaxially stretched polyester film according to the present invention will be described in more detail.

  The polyester used in the present invention is a polymer obtained by condensation polymerization from a diol and a dicarboxylic acid. Examples of the dicarboxylic acid include terephthalic acid, isophthalic acid, phthalic acid, naphthalenedicarboxylic acid, adipic acid, and sebacic acid. The diol is represented by ethylene glycol, trimethylene glycol, tetramethylene glycol, cyclohexanedimethanol and the like. Specific examples include polymethylene terephthalate, polyethylene terephthalate, polytetramethylene terephthalate, polyethylene-p-oxybenzoate, poly-1,4-cyclohexanedimethylene terephthalate, polyethylene-2,6-naphthalate, and the like. Of course, these polyesters may be homopolymers or copolymers, and examples of copolymer components include diol components such as diethylene glycol, neopentyl glycol, polyalkylene glycol, adipic acid, sebacic acid, phthalic acid, Examples thereof include dicarboxylic acid components such as isophthalic acid and 2,6-naphthalenedicarboxylic acid. In the case of the present invention, polyethylene terephthalate and polyethylene-2,6-naphthalate are particularly preferable from the viewpoint of mechanical strength, heat resistance, chemical resistance, durability, and the like. Among these, polyethylene terephthalate is preferable because of its low price. Further, particles may be added to the film raw material mainly composed of the polyester resin. The particles to be added are not particularly limited, and known additives such as heat stabilizers, oxidation stabilizers, weather stabilizers, ultraviolet absorbers, lubricants and antistatic agents can be used. However, it is preferable to consider the amount added depending on the application of the biaxially stretched polyester film.

  The raw material is dried and dehydrated, then supplied to a melt extruder and melt extruded under vacuum. The melted polyester is extruded into a sheet form from a molding die, and is brought into close contact with a casting drum as a cooling medium and rapidly cooled. Furthermore, it is preferable to make the film adhere to and solidify on the casting drum while applying an electrostatic charge from the viewpoints of suppressing crystallization of polyester, homogenizing the thickness, and preventing contamination of the casting drum surface.

  Next, the application of the coating liquid is not particularly limited, and when various coating liquids are applied for the purpose of imparting adhesiveness to one side or both sides of the film, for example, in the case of an optical film, an antireflection layer, This is desirable because the adhesiveness with a hard coat layer, a mat layer for light diffusion, etc. is improved. As the coating material, an aqueous solution such as polyester, acrylic polymer, polyamide, polyurethane, or an aqueous dispersion of resin is used. Here, although not particularly limited, a crosslinking agent may be added to such an extent that the effects of the present invention are not impaired in order to improve the coating film strength and stability. The cross-linking agent used is not particularly limited as long as it is a compound that causes a cross-linking reaction. Aziridine compounds, various silane coupling agents, various titanate coupling agents, and the like can be used. Further, in order to impart easy slipperiness, it is preferable to add inorganic particles to the coating liquid. As the inorganic particles to be added, typically, silica, colloidal silica, alumina, alumina sol, kaolin, talc, mica, calcium carbonate, or the like can be used. Although the compounding ratio with respect to solid content in a coating liquid is not specifically limited, 0.05-8 mass parts is preferable by mass ratio, More preferably, it is 0.1-5 mass parts. If the amount is less than 0.05 parts by mass, the slipperiness is hardly exhibited, and if it exceeds 8 parts by mass, the transparency of the film is deteriorated.

  As a method of applying the coating liquid thus obtained, first, before applying the coating liquid, it is effective to stabilize the application to perform treatment such as corona discharge treatment for improving the wettability of the film surface. Processing is performed accordingly, and then the above coating solution is applied to the film.

  In the production method of the biaxially stretched polyester film of the present invention, the film temperature when applying the coating liquid needs to be 65 ° C. or lower. The film temperature is preferably 55 ° C., more preferably 45 ° C. or less. When the film temperature exceeds 65 ° C., the coating liquid is solidified or agglomerated due to the heat of the film, and coating defects such as coating streaks and missing coating are generated.

  The method of cooling the film below the above temperature includes a method of lowering the film temperature by contacting the cooling roll again after passing through the casting drum, a method of passing through a water tank containing low-temperature cooling water, and cooling air on the casting drum. However, the method is not particularly limited thereto. A method (in-line coating method) in which the crystal orientation is completed by stretching and heat treatment after coating is preferably used from the viewpoint of cost.

  Although not particularly limited, such cooling of the film and application of the coating liquid can be performed in a step before stretching or a step after uniaxial stretching in the longitudinal direction.

  Moreover, the thickness of the film which apply | coats a coating liquid is 1000 micrometers or more. Preferably it is 1200 micrometers or more, More preferably, it is 1300 micrometers or more. A film thickness of 1300 μm or more is particularly preferable for obtaining the effects of the present invention because coating defects are unlikely to occur.

  As a biaxial stretching method, an unstretched polyester film is stretched in the longitudinal direction or the width direction, followed by sequential biaxial stretching in which stretching is performed in a direction perpendicular to the previous stretching direction, or once in the longitudinal direction and the width direction. In the present invention, the simultaneous biaxial stretching method is adopted.

  In the case of sequential biaxial stretching, the glass transition temperature of the resin is obtained by a known method such as a method in which the film prepared by the above method is brought into contact with several temperature-controlled rolls or a heating method using a radiant heat of an infrared heater or the like It heats to the above temperature, and extends | stretches in a longitudinal direction using the peripheral speed difference of the roll before and behind. The stretching ratio at this time is about 2 to 8 times, and the stretching may be performed in one step or stepwise in two or more steps. The film stretched in the longitudinal direction is once cooled and then stretched in the width direction by a stenter oven. The draw ratio in the width direction is drawn about 2 to 5 times. It is desirable that the stretched film is subsequently subjected to a heat treatment, and then the film is relaxed by several percent in the longitudinal direction and / or the width direction.

  In the case of simultaneous biaxial stretching, grip the above film with a clip, heat it above the glass transition temperature of the polyester in a stenter oven, and gradually increase the clip speed while gradually expanding the travel path of the clip. The longitudinal direction and the width direction can be stretched simultaneously. The film biaxially stretched by such a method can be subjected to heat treatment and relaxation treatment in the same manner as the sequential biaxial stretching described later. Films obtained by simultaneous biaxial stretching have less anisotropy and less distortion during heating compared to films obtained by sequential biaxial stretching, and there is an opportunity to contact the roll during the film production process. Since it is less than sequential biaxial stretching, there are few surface defects, and this is a particularly preferred stretching method.

  A method for measuring physical properties and a method for evaluating effects in the invention are shown below.

(1) Film temperature (° C)
The temperature of the film immediately before entering the coater is measured at an emissivity of 0.95 using a Keyence non-contact thermometer IT2-100. The measurement was performed about 200 mm and 500 mm inside from both ends of the film and the center in the width direction, and the average value was defined as the film temperature.

(2) Film thickness (μm)
The film immediately before entering the coater was sampled, and the thickness was measured with a micrometer (manufactured by Mitutoyo) at a pitch of 20 mm in the width direction, and the average value was taken as the film thickness.

(3) Coating defects The film after 10 hours from the start of coating was observed in a dark room with reflected light using a three-wavelength fluorescent lamp as a light source with a width of 1 m and a length of 20 m, and the number of coating defects was counted. The term “coating defects” used herein refers to “coating stripes” where there are portions that are not applied in the form of stripes in the longitudinal direction of the film, and “coating defects” in which uneven application occurs in the form of craters. The coating streaks were counted for those that could be visually confirmed, and the coating omission was counted for those having a diameter of 5 mm or more.

Example 1
Polyethylene terephthalate pellets substantially free of externally added particles are sufficiently vacuum dried, then supplied to an extruder, melted at 285 ° C., extruded into a sheet form from a T-shaped die, and the surface using an electrostatic application casting method The film was wound around a mirror casting drum at a temperature of 25 ° C., a diameter of 1500 mm, and a speed of 7 m / min to cool and solidify, and both sides of the unstretched film were subjected to corona discharge treatment in air. The film was passed through a water bath containing 20 ° C. cooling water for about 20 seconds and then conveyed to a bar coater. At this time, the temperature of the film was 33 ° C., and the thickness of the film was 1500 μm. The following coating solution was applied to both sides of the film with a bar coater. The unstretched film on which the coating solution has been applied is stretched simultaneously in the longitudinal and width directions by a known simultaneous biaxial stretching machine using a linear motor as a drive source for the clip, subjected to heat treatment, relaxed, and crystal oriented A biaxially stretched polyester film was formed. Film formation was continued in the above state, and a film after 10 hours from the start of application was evaluated by the above method. The results are shown in Table 1, and there were no coating streaks or coating omissions, and a very high quality film was obtained.

[Coating fluid] Emulsion of polyester copolymer copolymerized with the following composition
Terephthalic acid 50 mol%
Isophthalic acid 40 mol%
5-sodium sulfoisophthalic acid 10 mol%
Diol component Ethylene glycol 96 mol%
Neopentyl glycol 3 mol%
Diethylene glycol 1 mol%
Melamine crosslinking agent
A solution obtained by diluting an imino group-type methylated melamine with a mixed solvent of isopropyl alcohol and water (10/90 (mass ratio)),
A coating solution was prepared by adding 5 parts by mass of a melamine-based crosslinking agent and 1 part by mass of colloidal silica particles having an average particle size of 0.1 μm to 100 parts by mass of the above-described polyester resin.

(Examples 2 to 6)
A film was formed under the same conditions as in Example 1 except that the cooling method and conditions were changed. However, the film temperature and film thickness were as shown in Table 1. When the temperature of the film was within the range of the present invention, a good film was obtained both in coating stripes and coating omission.

(Example 7)
The polyethylene terephthalate pellets extruded from the die were cooled and solidified with a casting drum, passed through a roll and stretched in the longitudinal direction, and both surfaces of the film were subjected to corona discharge treatment in air. The film was passed through a water tank for about 20 seconds, and then the above coating solution was applied to both sides with a bar coater. At this time, the temperature of the film was 56 ° C., and the thickness of the film was 1350 μm. Subsequently, the film was gripped with a clip, stretched in the width direction with a known stenter oven, heat-treated, and relaxed to form a biaxially stretched polyester film having crystal orientation completed. The film after 10 hours from the start of application was evaluated by the above method. The results are shown in Table 1, and good films were obtained with both coating stripes and missing coating.

(Comparative Example 1)
A film was formed under the same conditions as in Example 1 except that the corona-treated film was not passed through the water tank. The film temperature and film thickness were as shown in Table 1. The resulting film had many coating streaks and coating omissions, and the quality was very poor.

(Comparative Example 2)
A film was formed under the same conditions as in Comparative Example 1 except that the film thickness was 1800 μm. The film temperature at this time was as shown in Table 1. The resulting film had many coating streaks and coating omissions, and the quality was very poor.

(Comparative Example 3)
A film was formed under the same conditions as in Comparative Example 1 except that the diameter of the casting drum was 1200 mm. The results are shown in Table 1, and there were many coating streaks and coating omissions, and the quality was very poor.

(Comparative Example 4)
A film was formed under the same conditions as in Example 1 except that the corona-treated film was not passed through the water tank. The film temperature and film thickness were as shown in Table 1. The resulting film had many coating streaks and coating omissions, and the quality was very poor.

Claims (1)

  A method for producing a biaxially stretched polyester film in which a coating liquid is applied to a film having a film temperature of 65 ° C. or less and a film thickness of 1000 μm or more by an inline coating method, and then the film is stretched by a simultaneous biaxial stretching method.